1. Field of the Invention
The invention relates to equipment for arc welding, and more specifically, it deals with an apparatus for overhead submerged-arc welding.
2. Description of the Related Art
A large number of operations are performed in the manufacture of welded structures with welding of rotatable annular joints of hollow products with a restricted access to joints being welded from the interior. Such joints include annular joints of closed vessels, annular joints of pipelines, tanks, casings, assembly welds, shell plating seams of ship hulls; longitudinal welds of large-area products which it is difficult to place to a position facilitating welding. Such joints include joints between difficult-to-position webs, segments, three-dimensional and planar sections and other members.
The overhead submerged-arc welding method is characterized by the fact that a consumable electrode and welding bath are turned at 180.degree. in comparison with the downhand submerged-arc welding. Flux and electrode are supplied from bottom up, i.e. as though towards a ceiling. The electrode is supplied through compacted flux.
This is why this welding method will be referred to hereinbelow as overhead submerged-arc welding.
This welding method is referred to as the overhead submerged-arc welding also because arc is in the body of metal.
So called overhead welds are produced as a result of such welding.
Overhead welds may be of various types, e.g. penetration overhead welds and sealing overhead welds. There may also be one-pass overhead welds and other overhead welds.
The penetration overhead welds are the welds which are first to be produced in welding a joint and which are located in the top part of sections being welded, on the opposite side of the joint with respect to the electrode supply. Further welding of the joint, i.e. producing further welds, can be carried out by any appropriate known method, the electrode being supplied on the same side as was the case in producing the penetration overhead weld, e.g. producing inner penetration welds of rotatable annular joints of vessels, tanks, joints between bottom sections of shell plating of ships, and other structures.
The overhead submerged-arc welding of penetration welds mainly allows welding inside vessels in producing rotatable annular welds to be eliminated, and welding can also be avoided in confined spaces in producing straight welds of structures with an access on the side opposite to the ceiling.
The sealing overhead welds are the welds which are first to be produced in welding a joint and which are located in the bottom part of sections being welded on the joint side in the vicinity to the electrode supply. Further welding is carried out by any appropriate known method, the electrode being supplied on the opposite side of the joint as compared with overhead welding.
In practice, the penetration overhead welds are produced in welding annular and longitudinal joints of structures with a restricted access to joints being welded from the interior.
The sealing welds are produced in welding elongated longitudinal joints of difficult-to-position products such as plate structures made out of segments or other members.
The one-pass overhead welds are the welds produced in welding joints of a limited thickness located over the whole welded section. No further welding of the joint on either side is required.
Many problems arise in producing sealing and one-pass welds in forming the surface of the finished weld.
The metal in the welding bath formed during arcing by fusion of the metal being welded, electrode material and welding flux is held by the crust of partly melted flux and by forming means. Forming means may be of various configuration and size and may be, e.g. in the form of plates, backings, bars, sliders and other members.
Flux is positively pressed against the welding spot from bottom, and as flux is consumed, its stock is continually replenished. Flux may be supplied for forming the top part of the weld either on the bottom side through the gap between the edges of welded members or from top by any appropriate known method so as to form a filled flux layer. Special forming backings or flux holders may also be used.
Special problems arise in welding joints of large-size cylindrical or like products such as ship hulls and boiler units where especially high quality of welds is required and where the product should be rotated about its axis during welding, and also in welding large-size planar members which it is difficult to place to a position facilitating welding.
Known in the art is an apparatus for overhead submerged-arc welding (SU,A,1348111), comprising a hopper containing flux and accommodating a bowl and a welding nozzle for a consumable electrode extending through the hopper and bowl.
The hopper supports a welding head. The hopper is pivotally mounted on a suspension for rotation about a pivot pin of the suspension. The same pivot pin supporting the hopper also supports a forming means and a copying member in the form of a copying wheel. The apparatus has means for turning the hopper about its support pivot pin and a means for pressing the forming means with the copying member against a work being welded. The support pivot pin of the hopper is aligned with the upper end face of a rear wall of the bowl. The forming means is mounted in such a manner that the main part thereof is received in the bowl above the brim of an outlet port thereof facing towards the work being welded. The pivot pin supporting the hopper is mounted on a mounting arm provided on a carriage which has the means for pressing the forming means with the copying member against the work being welded, comprising a power actuator for moving the carriage in the direction towards the work being welded.
The support pivot pin is mounted on the hopper in an offset position with respect to a means for supplying flux and pressing it against the work being welded so that the hopper supported by this pivot pin forms a double-arm lever, one arm of the lever supporting the means for supplying flux and pressing it against the work being welded and the other arm being connected to the means for turning the hopper about the support pivot pin. The means for turning the hopper is in the form of a power actuator pivotally connected to the hopper and carriage.
The hopper suspension is mounted on a driven trolley which is adapted to move in the welding direction.
The construction of the prior art apparatus makes it possible to vary both absolute values of flux pressure in the bowl and force with which the forming means is pressed against the work being welded and the ratio therebetween.
This apparatus allows welding to be carried out with the desired weld formation on either side thereof in a broad range of process capabilities with a wide range of welded structures.
However, as the forming means and the copying wheel in this apparatus are mounted on one and the same pivot pin aligned with the upper end face of the rear wall of the bowl which is stationary, the forming means is pressed away from the work in cases of substantial convexities of the joint being welded on the underside thereof. This results in a substantial change in position of the axis of oscillations of the hopper and copying wheel with respect to the surface of the work being welded, hence, in an abrupt change in preset flux pressures in the bowl at various points of formation of the welding bath (upstream of the arc, in the zone of the arc and welding bath, and at the point of solidification of the welding bath), i.e. in a disruption of welding as a whole and in impaired quality of welded joints.
In case of a substantial concavity of a joint being welded on the underside of the work, the forming means is separated therefrom, and a substantial additional space is formed between the working face of the forming means and the work so that welding is also disrupted.
In addition, the provision of the main part of the forming means inside the bowl, in the area of strongly compacted flux filling it during welding, hampers oscillations of the forming means with respect to its pivot pin thus hampering copying of the surface of the work and disrupting conditions for maintaining a constant preset flux pressure in the bowl, hence, resulting in a lower welding quality as a whole.
Also known in the art is an apparatus for overhead submerged-arc welding (DE,C, 3430394), comprising a hopper containing flux and accommodating a bowl having inlet and outlet ports. The inlet port of the bowl communicates with a supply pipe which house an auger connected to a drive. The outlet port of the bowl faces towards a work being welded. A welding nozzle for a consumable electrode extends through the hopper and the bowl. The hopper supports a welding head and an auger drive.
The hopper is pivotally mounted on a mounting arm for rotation about its pivot pin.
The pivot pin supporting the hopper is mounted on the mounting arm which is movable in the direction towards the work being welded. This mounting arm also supports a forming means and a copying member which are attached thereto by means of pivot pins and suspensions.
The forming means is mounted for oscillations in its transverse and longitudinal planes. A transverse bar is provided at the front edge of the forming means located adjacent to the welding nozzle on the bottom face of the forming means, the transverse bar cooperating with the rear wall of the bowl during welding.
The copying member engages the surface of the work being welded during welding. The support pivot pin of the forming means is in the form of a point-like abutment at the end of a double-arm lever having its fulcrum attached to a mounting arm, the other arm of the lever being connected to another power actuator pivotally connected to the mounting arm. The copying member is in the form of a copying wheel and is mounted on the pivot pin supporting the hopper. The pivot pin supporting the forming means in another embodiment of the apparatus is in the form of a point-like abutment at the end of a mounting arm adjacent to the welding nozzle. In this embodiment of the apparatus, the copying member is in the form of projections on the surface of the forming means, the pivot pin supporting the hopper is located on the side of the forming means remote from the nozzle, and the hopper has a means for moving it with respect to its support pivot pin.
In this apparatus, the mounting arm is mounted on a carriage which has a drive for moving it and is mounted on a driven trolley for moving the whole apparatus in the welding direction.
This apparatus allows permanent contact between the forming means and copying member on the one hand and the work being welded on the other hand to be ensured during welding with various errors of assembly before welding and of geometry of the joint being welded (e.g. misalignment of plate edges, convexity and concavity, undulations, clearances, and the like).
Therefore, upon any change in position of the forming means during welding caused by an admissible change in profile of the surface of the work being welded at point of their contact the possibility of the forming means acting upon position of the hopper containing flux is prevented. This facility prevents the hopper from performing undesired oscillations upon changes in profile of the work surface and stabilizes welding parameters such as flux backing thickness and flux pressure in the welding zone.
The flux backing is an area of a compacted compressed flux layer which has a preset pressure distributed over the whole area of the flux backing and which is located between the top part of the bowl facing towards the work being welded and the surface of the work being welded to exert a local pressure upon the surface of the joint being welded in the welding zone.
This construction of the apparatus provides conditions for a smooth copying of the surface of the joint being welded by the forming means without jerks and shakes which is necessary for maintaining stable preset values of flux pressure acting upon the welding bath and upon zones in which the welding bath is formed along the joint being welded.
The construction of the prior art apparatus also provides conditions for a smooth copying of the surface of the joint being welded by the forming means and for adjusting position of the hopper with respect to the work being welded which is necessary for carrying out welding of products of different configurations.
In this apparatus, a change in position of the forming means during welding which occurs because of errors of geometry and assembly of the joint being welded does not cause a change in position of the pivot pin supporting the hopper with respect to the surface of the work being welded.
In welding with such an apparatus, owing to the creating and maintenance at a constant level of preset flux pressures at various points along the joint being welded (upstream of the arc, in the arc and welding bath zone, and in the zone downstream of the welding bath and up to the formed weld), the possibility of automatic conduct of welding of overhead welds and production of high-quality welded joints is ensured.
This apparatus also ensures high-quality overhead welding in a broad range of process capabilities.
However, in welding products with large assembly errors of joints before welding or with local changes in geometry of joints being welded (e.g. in welding thin-walled bottles, receivers, and the like), when the forming means substantially changes its position with respect to the work, flux can be jammed in the space between the rear wall of the bowl and the bar at the bottom of the forming means. This jamming of flux results in the bowl being pressed away, together with the hopper, from the surface of the work being welded; an additional space is formed between the bowl and work, hence, the preset flux pressure in the zone of the electrode and at various points along the joint being welded is changed thus bringing about disruption of welding and lower welding quality over a broad range of process conditions.
In addition, the pivot pin supporting the hopper and the pivot pin supporting the forming means in this apparatus are spaced from each other which, in welding the abovementioned products having errors of geometry and assembly of joints, causes a substantial relative movement of the forming means with the transverse bar located at its bottom and the rear wall of the bowl so as to intensity jamming of flux in the clearance between the rear wall of the bowl and the transverse bar and result in fluctuations of the preset pressure of flux in the bowl and in a lower welding quality.